cmake finds wrong python libs

Question

I'm new to CMake and have trouble understanding some usage concepts.

I'm calling a python script from a c++ program:

#include <Python.h>
...
Py_Initialize();
PyRun_SimpleFile(...);
Py_Finalize();

The corresponding cmake entries in my cmake file are:

FIND_PACKAGE(PythonLibs REQUIRED)
...
TARGET_LINK_LIBRARIES(MyApplication ${PYTHON_LIBRARIES})

This works as long as my python script isn't using any modules installed into the site-packages directory, otherwise I get an ImportError. This question shows how to find the location of the site-packages directory with CMake, but what should I tell CMake to do with it?

EDIT: Problem solved. Turns out FIND_PACKAGE(PythonLibs) finds a different python installation from what I'm normally using (/usr/local/lib/libpython2.7.dylib instead of /Library/Frameworks/Python.framework/Versions/2.7/lib/libpython2.7.dylib - I'm on mac), which is how I get standard python modules, but none that I installed myself. To change the PYTHONPATH back to normal, I added

try:
  import some_package
except ImportError:
  if "my_python_path" in sys.path: raise
  sys.path.append("my_python_path")

at the top of my python script.

Answer 1

You can tell cmake where to find this PythonLibs by specifying the path to your python libraries like this:

cmake -DPYTHON_LIBRARIES=/Library/Frameworks/Python.framework/Versions/2.7/lib/libpython2.7.dylib .

This will then set the ${PYTHON_LIBRARIES} inside cmake to the right path.

To find out which other possible options (besides PYTHON_LIBRARIES) you can give to cmake (with the -DARG option) try running

ccmake .

Then press c to configure, and t for advanced options.

For example, you might also want to set

-DPYTHON_LIBRARY='/softwarepath/Python/Python2.7/lib/libpython2.7.so'
-DPYTHON_INCLUDE='/softwarepath/Python/Python2.7/include'

Answer 2

The best way to solve the problem that the wrong version is found (for instance 3.0 instead of 2.7) is to specify the minimum version to find_package (this will choose any version >= 2.7):

FIND_PACKAGE(PythonLibs 2.7 REQUIRED)

or to get the exact version:

FIND_PACKAGE(PythonLibs 2.7.5 EXACT REQUIRED)

Answer 3

You can setup manually on cmake libs \usr\share\cmake-3.2.3\Modules\FindPythonLibs.cmake:

set(PYTHON_LIBRARY "\\usr\\lib\\python2.7")
set(PYTHON_INCLUDE_DIR "\\usr\\include\\python2.7")